1. Field of the Invention
The present invention relates generally to a serial data receiving device in which an input serial bit string distorted due to noise is correctly received according to an over-sampling method.
2. Description of Related Art
In a case where binary digits are sampled from an input serial bit string received in a conventional serial data receiving device, a level change time (hereinafter called a change point) of each binary digit of the bit string is unknown in the conventional serial data receiving device. Therefore, in a case where a binary digit is undesirably sampled in the neighborhood of its change point, there is a probability that a value of the sampled binary digit is incorrect.
To prevent the occurrence of error in the sampling for the input serial bit string, an over-sampling method has been adopted to correctly perform the sampling of binary digits.
In detail, as shown in FIG. 7, binary digits are sampled from an input serial bit string at a sampling rate higher than a bit rate of the binary digits. For example, the sampling is performed five times during a time period corresponding to one binary digit to obtain five sampled binary digits for each binary digit. Therefore, an amount of sampled binary digits is five times larger than an amount of binary digits of the input serial bit string. Though there is a probability that data sampled at positions near change points is incorrect, there is a high probability that data sampled at positions far from change points is correct. In the over-sampling method, data correctly sampled at a high probability is selected from five sampled binary digits corresponding to each binary digit of the bit string, and a set of parallel bit strings composed of pieces of data correctly sampled at a high probability are obtained from the input serial bit string.
FIG. 6 is a block diagram showing the configuration of a conventional serial data receiving device. In FIG. 6, 101 indicates an over-sampling unit for performing the over-sampling N times (for example, N=5) for each input binary digit of an input serial bit string to obtain a serial bit string of over-sampled binary digits from the input serial bit string. 102 indicates a serial-parallel converting unit for converting a plurality of over-sampled binary digits corresponding to each group of L input binary digits (for example, L=10) successively placed in the input serial bit string to a set of L parallel bit strings respectively composed of N over-sampled binary digits. 103 indicates a data selecting unit for selecting a correctly-sampled binary digit from N over-sampled binary digits of each parallel bit string to obtain a selected bit string composed of L correctly-sampled binary digits and to obtain a plurality of selected bit strings corresponding to the input serial bit string. 104 indicates a word retrieving unit for retrieving a string of words from the correctly-sampled binary digits of the selected bit strings.
Next, an operation of the conventional serial data receiving device will be described below.
FIG. 7 is an explanatory view of the over-sampling method performed in the conventional serial data receiving device for an input serial bit string. In this example, the number of over-sampling operations for each input binary digit is set to five.
When an input serial bit string composed of input binary digits serially arranged is received in the over-sampling unit 101, the sampling is performed five times for each input binary digit of the input serial bit string to obtain a serial bit string of over-sampled binary digits, and a plurality of over-sampled binary digits corresponding to each group of input binary digits successively placed in the input serial bit string are converted in the serial-parallel converting unit 102 to a set of parallel bit strings respectively composed of five over-sampled binary digits. The number of parallel bit strings is equal to the number of input binary digits corresponding to the conversion, and each parallel bit string corresponds to one input binary digit. In this example, fifty over-sampled binary digits corresponding to each group of ten input binary digits successively arranged in the input serial bit string are converted to a set of ten parallel bit strings composed of fifty over-sampled binary digits (5xc3x9710=50). Therefore, an operating frequency of the conventional serial data receiving device can be reduced, each group of ten input binary digits included in the input serial bit string can be estimated at once, and the data selection and the word retrieval can be easily performed later.
Thereafter, in the data selecting unit 103, ten over-sampled binary digits correctly sampled at a high probability are selected from each set of ten parallel bit strings as a selected bit string of ten correctly-sampled binary digits. In this case, to select one over-sampled binary digit correctly sampled at a high probability from each parallel bit string, a method for finding out two successive over-sampled binary digits having different values at a high probability is often used. For example, a value of the first over-sampled binary digit differs from a value of the second over-sampled binary digit at a high probability in each of four parallel bit strings xe2x80x9c10000xe2x80x9d, xe2x80x9c01111xe2x80x9d, xe2x80x9c11111xe2x80x9d and xe2x80x9c01000xe2x80x9d shown in FIG. 7. In this case, it seems reasonable to conclude that the change point of each input binary digit in the input serial bit string is placed between the first over-sampled binary digit and the second over-sampled binary digit of the corresponding parallel bit string. Also, because the fourth over-sampled binary digit has the same value as those of the third and fifth over-sampled binary digits adjacent to the fourth over-sampled binary digit in each parallel bit string at a high probability, it seems that the value of each fourth over-sampled binary digit is stable. Therefore, the fourth over-sampled binary digit in each of ten parallel bit strings is selected as a correctly-sampled binary digit. Because the transmission time period for each input binary digit of the input serial bit string is normally constant, the change point of the input binary digit is placed every five over-sampled binary digits and is placed between the (5M+1)-th over-sampled binary digit and the (5M+2)-th over-sampled binary digit (M denotes zero or a positive integral number), and one (5M+4)-th over-sampled binary digit is to be selected as a correctly-sampled binary digit from each of ten parallel bit strings. In the example shown in FIG. 7, the fourth (M=0 in (5M+4)-th) over-sampled binary digit, the ninth (M=1 in (5M+4)-th) over-sampled binary digit,---, the forty-ninth (M=9 in (5M+4)-th) over-sampled binary digit are selected as ten correctly-sampled binary digits of one selected bit string.
However, in the actual operation, the input serial bit string undesirably receives the influence of noises, the change point of an input binary digit is moved, for example, from the position between the (5M+1)-th over-sampled binary digit and the (5M+2)-th over-sampled binary digit to the position between the (5M+2)-th over-sampled binary digit and the (5M+3)-th over-sampled binary digit. In this case, to reliably select correctly-sampled binary digits, one over-sampled binary digit most stable among five over-sampled binary digits in each parallel bit string is selected as one correctly-sampled binary digit.
Thereafter, a plurality of selected bit strings respectively composed of ten correctly-sampled binary digits are successively input to the word retrieving unit 104. In general, a group of input binary digits preset to a predetermined bit number and expressing a fixed meaning is included many times in the input serial bit string. For example, in case of picture data, red information of one pixel, green information of one pixel and blue information of one pixel are respectively expressed by using a group of eight (or ten) binary digits. Therefore, when picture data is transmitted as an input serial bit string, a serial data receiving device is required to divide the input serial bit string every eight binary digits denoting a unit of pixel information. A bit string of input binary digits expressing a meaning is called a word. In the word retrieving unit 104, a boundary position between each pair of words is found out, the correctly-sampled binary digits of the selected bit strings are divided at the boundary positions, and a bit string of correctly-sampled binary digits corresponding to each word is output.
To easily find out a boundary position between each pair of words, a predetermined bit string preset to a predetermined bit number is many times included in the input serial bit string, and the input serial bit string including the predetermined bit strings is transmitted to a serial data receiving device during a prescribed time period. This predetermined bit string is called a referential word, and referential words included in the input serial bit string are transmitted in a referential word transmitting time period. In the word retrieving unit 104, the selected bit strings respectively composed of correctly-sampled binary digits are searched for one predetermined bit string expressing one referential word. In cases where the predetermined bit string expressing one referential word exists in the selected bit string of correctly-sampled binary digits, a boundary position between each pair of words can be found out by detecting the predetermined bit string expressing one referential word.
A referential word retrieving method will be described in detail below. For example, a bit number of each referential word is set to ten, and a bit number of each word desired to be transmitted is also set to ten. In the word retrieving unit 104, twenty correctly-sampled binary digits of each set of two selected bit strings output from the data selecting unit 103 are stored in a storing circuit having an area of twenty bits. The group of twenty correctly-sampled binary digits included in each set of two bit strings is searched for a referential word composed of ten over-sampled binary digits. As a searched result, in a case where it is found out that a group of ten over-sampled binary digits placed from sixth bit to fifteenth bit agrees with one referential word, the referential word is detected in the group of twenty correctly-sampled binary digits, and a starting point of the referential word placed at the sixth bit is detected. After the transmission of the input serial bit string including the referential words in the referential word transmitting time period, the input serial bit string not including any referential word but including desired words is transmitted to the serial data receiving device. In the word retrieving unit 104 of the device, a group of ten correctly-sampled binary digits starting from the sixth bit of twenty correctly-sampled binary digits in each set of two selected bit strings is retrieved as one word, and a string of words successively arranged is output.
As is described above, in the conventional serial data receiving device, the over-sampling is performed N times for each input binary digit of the input serial bit string each time L input binary digits of the input serial bit string is received, L correctly-sampled binary digits are selected from L sets of N over-sampled binary digits, and words are retrieved from the correctly-sampled binary digits. In the selecting processing of the correctly-sampled binary digits, each selected bit string of L correctly-sampled binary digits is determined from L parallel bit strings of N over-sampled binary digits by finding out the change point of the N over-sampled binary digits in each parallel bit string. However, in a case where the change point of the N over-sampled binary digits cannot be determined, it is required to select each correctly-sampled binary digit from each parallel bit string of N over-sampled binary digits according to statistical analysis. Therefore, in a case where the input serial bit string is considerably distorted due to the influence of noise, there is a probability that one over-sampled binary digit not correctly sampled is erroneously selected as one correctly-sampled binary digit to produce an incorrect bit string, and the word retrieving operation for the referential word is performed for the incorrect bit string of 2*L correctly-sampled binary digits. In this case, no bit string agreeing with a referential word is found out from a bit string of 2*L correctly-sampled binary digits including the incorrect bit string. As a result, a problem has arisen that a string of words cannot be correctly received or output.
An object of the present invention is to provide, with due consideration to the drawbacks of the conventional serial data receiving device, a serial data receiving device in which a string of words is correctly received and output even though input serial data is considerably distorted.
The object is achieved by the provision of a serial data receiving device including over-sampling means, serial-parallel converting means, data dividing means, data retrieving means and output data selecting means. In the over-sampling means, a sampling operation is performed N-th times for each input binary digit of an input serial bit string having a referential word, and a plurality of over-sampled binary digits are produced. In the serial-parallel converting means, an over-sampling bit string of the over-sampled binary digits is converted to a plurality of parallel bit strings respectively corresponding to one input binary digit of the input serial bit string. In the data dividing means, the N over-sampled binary digits of each parallel bit string are divided to N groups to produce N divided bit strings corresponding to the N groups of sampling operations. In the data retrieving means, a plurality of divided bit strings having the referential word are detected, a group specified result indicating a plurality of groups corresponding to the divided bit strings having the referential word is output, and a string of desired data is retrieved from each divided bit string having the referential word. In the output data selecting means, one group is selected as a final selected group from the groups of the group specified result, and the string of desired data corresponding to the final selected group is output as an output bit string.
Accordingly, even though the input serial bit string is considerably distorted, a string of words representing the string of desired data can be correctly retrieved from the input serial bit string and can be output.